Activated nickel screens are currently being used for the synthesis of methane and the generation of hydrogen and oxygen in electrolytic cells containing an aqueous alkaline medium. In methane synthesis a mixture of carbon monoxide and hydrogen are passed over the activated nickel screens to form methane and water. In the production of hydrogen and oxygen in electrolytic cells, the activated nickel screens are used as the cathode. The activated screens, when used as the cathode in an electrolytic cell, lower the overvoltage and show more than a 20% improvement in efficiency over untreated nickel screens. It is believed that the superiority of the activated nickel screens is due, at least in part, to the increased surface area that results from the activation step. The activated screens have been used in electrolytic cells for the generation of hydrogen and oxygen for about ten years.
Hydrogen is presently being used as a fuel for industrial applications as well as a fuel for automobiles. The advantage of hydrogen as an automobile fuel include a greater energy release per unit weight of fuel and the absence of polluting emissions including carbon monoxide, carbon dioxide, nitrogen oxide, sulfur oxides, hydrocarbons, aldehydes, and lead compounds (i.e., the combustion products of hydrogen are primarily water with minute traces of nitrogen oxide).
The known process to produce the activated nickel screens included placing each individual nickel screen in a "pack" composed of a powder mixture containing aluminum, aluminum oxide and a halide salt activator followed by a heating operation (i.e., for several hours at elevated temperatures). This is known as the Classical Pack Cementation process and is disclosed in U.S. Pat. No. 4,349,612. The chemistry of this process during the heating step includes the reaction of the halide with aluminum to yield gaseous aluminum sub halide such as aluminum sub chloride (AlCl). As this gas passes over the nickel screen, it decomposes and deposits aluminum on the nickel surface. The process is carried out for 20 to 30 hours at 800-1200.degree. F. in a hydrogen atmosphere. At this temperature the deposited aluminum diffuses into the nickel surface to form a coating comprising an aluminum rich nickel aluminide (Ni.sub.2 Al.sub.3). The process is labor intensive, requires long processing times, gives off obnoxious dusts during loading of the screens and emits corrosive and toxic halide gases during the heating operation. In order to prevent contamination of the environment, the effluent gases must be scrubbed under alkaline conditions to neutralize and remove the toxic gases. In addition, after each processing cycle, the coating powder must be sifted and replenished for the next load of screens. The powder mixture is sensitive to water absorption and must be kept dry when not in use. Otherwise the moisture will react with the activator in the pack and curtail its function.
After the formation of the nickel aluminide coating on the nickel screens, the screens are immersed in a 20% solution of sodium hydroxide for about 40-60 minutes at 180-200.degree. F. to selectively leach out at least a portion of the aluminum from the nickel aluminide coating. The screens are then rinsed in water and passivated by immersion for one hour in hot water at 180 to 212.degree. F. followed by a one hour immersion in a water solution containing 2-5% hydrogen peroxide at 74.degree. F. followed by rinsing in water and finally drying in an oven at 140-160.degree. F. to remove all water from the screen. After the foregoing processing, the screens are ready to be used as cathodes in electrolytic cells containing an aqueous alkaline medium (for example, 25% NaOH or 25% KOH in water). In these electrolytic cells, hydrogen is produced at the cathode and oxygen is produced at the anode. The anodes of the cells are usually composed of virgin (untreated) nickel. It is preferred that the anodes contain pores or openings (e.g., nickel screen).